1. Field of the Invention
This invention relates in general to electric variable speed drives for variably coupling a mechanical load to a prime mover, and in particular to shaft mounted eddy current drive for providing a variable magnetic coupling between a driven member and a prime mover.
2. Description of the Prior Art
Electric variable speed drives, and in particular eddy current drives, have been used to selectively determine the extent of coupling between a prime mover and a mechanically driven load. One example of an electric variable speed drive is the shaft mount eddy current drive disclosed in U.S. Pat. No. 4,400,638, invented by James W. Albrecht, et al., and issued to Stromag, Inc., on Aug. 23, 1983.
Eddy current drives effect a variable magnetic coupling between an input member and a driven member by means of an electric coil, which is mounted for rotating with one of the input and driven members. The input member and shaft of the prime mover are coupled for rotating at fixed speeds. The driven member includes a mechanical output drive portion which is directly coupled to the mechanically driven load for operating the load at speeds which correspond to speeds of the driven member. The speeds at which the driven member rotates are determined by the extent of magnetic coupling between the input member and the driven member, and the extent of the magnetic coupling is determined by electric current passing through the electric coil.
Prior art eddy current drives typically have solid brushes and slip rings for passing the electric current from a stationary power source to the rotating electric coil. The slip rings, or commutator rings, are mounted for rotating with the member which includes the electric coil. The solid brushes, which are usually made of carbon, are held in a stationary position and pressed against the rotating slip rings to electrically connect between the electric coil to the stationary power source. Force must be applied to press to the solid brushes against the slip rings to assure that an adequate electrical connection is maintained.
Pressing solid brushes against rotating slip rings causes the brushes to wear and require replacement. Excessive force may also deteriorate slip rings. Thus, prior art eddy current drives having solid brushes and slip rings require periodic maintenance to replace worn brushes and repair damaged slip rings. Often, brushes have to be replaced after only several months of use.